1. Field of the Invention
The present invention relates to an image-forming process and an image-forming apparatus for forming a full-color toner image by supplying color toners onto a recording medium and then forming the full-color image by fixing the toner image formed on the recording medium by using a flash fusing device, either in an electrophotographic process, an electrostatic recording process, a magnetic recording process, or the like.
2. Description of the Related Art
In electrophotographic processes commonly employed in copying machines, printers, printing machines, and the like, images are generally formed in the following manner: the photoconductive insulator surface of a photoreceptor drum is first uniformly charged positively or negatively (in a charging step), and then an electrostatic latent image is formed according to image information by irradiating, for example, a laser beam onto the photoconductive insulator surface and thus partially removing the electrostatic charge on the insulator surface. The latent image is then converted to a visible toner image, for example, by applying fine particles of a developer called toner onto the latent image area retaining the electrostatic charge on the photoconductive insulator. Generally, the toner image obtained in this manner is transferred electrostatically onto a recording medium such as recording paper and then the toner image is fixed on the recording medium in order to produce printed matter.
Various solidification and fusion methods including fusion of the toner by application of heat and/or pressure and fusion of the toner by irradiation of light energy have been used for fixing the toner image after transfer, and flash fusing processes (also called flash fixing processes) utilizing light, which are advantageous compared with application of heat or pressure, are now attracting more attention.
That is, the flash fusing process, which demands no pressure for toner fixation, has an advantage that the resolution (reproducibility) of the toner image is less deteriorated in the fixing step because the image needs not be brought into contact (or pressurized) with, for example, a fixing roller. In addition, such a device allows printing immediately after it is turned on, because it demands no preheating of heat sources such as a fixing roller and thus eliminates the waiting time for the heat sources to be preheated to a desired temperature after it is turned on. Elimination of the high-temperature heat sources is also advantageous in effectively preventing the rise in temperature of the device and in preventing the ignition of recording paper due to the heat from the heat sources even when the recording paper clogs in the fixing device due to system malfunction.
However, when color toners are used for fixing, the flash fusing process is rather lower in fixing efficiency than when a black toner is used, because of the lower light absorption efficiency of the color toners. Accordingly, many methods for improving the fixing efficiency by adding an infrared absorbent to the color toner have been proposed (e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 60-63545, 60-57858, 60-131544, 61-132959, 6-348056, 7-191492, 10-39535, 11-38666, 11-65167, 11-125930, 2000-147824, 2000-155439, and 2000-35689). These proposed methods aimed at eliminating the problem of the deterioration in toner fusion properties and thus establishing well-balanced multicolor printing and flash fusing efficiencies, by adding to a toner a material absorbing light in the infrared region as an infrared absorbent. These methods also aimed at improving fixing efficiency by increasing the light intensity during flash fusing at the same time.
However, while the increase in the intensity of the light used for fixing leads to improvement in fixing efficiency it also causes printing defects; namely, “voids” formed in the toner image by evaporation of water and the like from the toner and the recording medium. It is therefore necessary to optimize the balance between the light intensity during image fixation and the toner fusion properties in order to satisfy both improved fixation and the margin for prevention of void generation. In particular if plural kinds of toners are superimposed and then fixed all at once by light, the increase in the amount of the toners applied onto a recording medium leads to a decline in toner fixing efficiency. Further an increase in the amount of toners deposited on a recording medium results in increase in the void generation rate during image fixation. It is more difficult to satisfy both favorable fixing efficiency and void resistance at the same time when the toner layer becomes thicker.
Alternatively, the order of superimposing the respective toner layers forming a full-color toner image has been investigated with a view to improving the flash fusing efficiency of toner. For example, a method of forming the top layer by using a yellow toner, which is usually lowest in fixing efficiency, has been proposed as the order of superimposing the toner layers (e.g., JP-A No. 2002-174924), but this condition is unfavorable because the fixing efficiency during multi-color printing deteriorates and use of an infrared absorbent in an increased amount for fixing the yellow toner results in turbid color. In addition, an increase in fixing light energy leads to an increased amount of voids, thus prohibiting favorable fixation and getting the enough margin for the void prevention.
Alternatively, the photoacoustic spectrometric (PAS) intensity of a light having a wavelength in the range of 800 to 2,000 nm has been investigated (e.g., JP-A Nos. 2003-295496 and 2003-295497). In these disclosures, it was proposed that a light having a higher PAS intensity should be irradiated to the top-layer toner during flash fixation. However, the intensities of the toners absorbing a flash light differ significantly depending on the colorants used: cyan, magenta, or yellow. For example, if a cyan pigment absorbing the light in a greater amount is used, the fixing efficiency of the image is higher even when it is irradiated with a light having a lower PAS intensity. The fixing efficiency of the toner image therefore could not necessarily be discussed only in relation to the PAS intensity. Further, generation of the voids and deterioration in the surface smoothness of fixed images remains even when using this method.